1. Field of the Invention
This invention relates to a magnetic memory device and a method for manufacturing the same which can be applied to an MRAM or the like, for example.
2. Description of the Related Art
A magnetic random access memory (which is hereinafter simply referred to as an MRAM) is a general name of a solid-state memory which can rewrite, hold and read out memory information at any time by using a magnetization direction of a ferromagnetic body as an information storage medium.
The memory cell of the MRAM generally has a structure in which a plurality of ferromagnetic members are laminated. The information recording operation is performed by setting binary information item “1” or “0” to respectively correspond to the state in which the relative arrangement of magnetization of a plurality of ferromagnetic members is set to the parallel or anti-parallel arrangement. The memory information writing operation is performed by inverting the magnetization direction of the ferromagnetic members of each cell by use of a current magnetic field generated by causing a current to flow into one of write lines arranged in a cross stripe form. The MRAM is a nonvolatile memory in which the power consumption at the storage information holding time is principally zero and the storage information holding operation is performed even after the power supply is cut off. The memory information readout operation is performed by utilizing a so-called magneto-resistance effect or a phenomenon that the electrical resistance of the memory cell varies according to a relative angle of the magnetization direction of the ferromagnetic member configuring the cell with respect to a sense current or a relative angle of magnetization between a plurality of ferromagnetic layers.
For example, the MRAM is advantageous in the following respects when the function thereof is compared with that of the conventional semiconductor memory using a dielectric member.
(1) The MRAM is a complete nonvolatile memory and the rewriting operation can be performed by 1015 times or more.
(2) The non-destructive readout can be performed and the refresh operation is not required so that the readout cycle can be shortened.
(3) The resistance to radioactive rays is strong in comparison with that of the charge storage memory.
It is predicted that the integration density per unit area, writing time and readout time of the MRAM can be set approximately equal to those of the DRAM. Therefore, it is expected that the MRAM can be applied to an external memory device of a mobile equipment, LSI hybrid device or a main storage memory of a personal computer by making use of the significant feature of the nonvolatile characteristic.
As the MRAM which is studied for practical application at present, an MTJ element using ferro-magnetic tunnel junction (which is hereinafter referred to as MTJ) in a memory cell is provided. The above MRAM is described in ISSCC 2000 (Digest Paper TA7.2 A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell).
The MTJ element has a three-layered structure mainly formed of a ferromagnetic layer/insulating layer/ferromagnetic layer and a current tunnels and flows through the insulating layer. The junction resistance varies in proportion to cosine of a relative angle of magnetizations of the two ferromagnetic metal layers and is set to a maximum value when the magnetizations are set anti-parallel. This is the tunnel magneto-resistance effect and a magnetic resistance variation rate exceeding 25% is observed in a weak magnetic field of 50 Oe or less in NiFe/Co/Al2O/Co/NiFe, for example. As the structure of the MRAM, a coercive force difference type structure which holds data by use of a difference of coercive forces of the two ferromagnetic layers is known. Further, a so-called spin valve structure in which an anti-ferromagnetic member is arranged adjacent to one of the ferromagnetic layers and the magnetization direction is fixed in order to improve the magnetic field sensitivity or reduce a write current is known. The above MRAM is described in Jpn. J. Appl. Phys. Vol. 36, L200, 1997 (Spin-Valve-Like Properties of Ferromagnetic Tunnel Junction).